On 28/08/2024 18:27, Boris Brezillon wrote:
On Wed, 28 Aug 2024 18:07:03 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx> wrote:
On Wed, 28 Aug 2024 14:22:51 +0100
Mihail Atanassov <mihail.atanassov@xxxxxxx> wrote:
Hi Boris,
On 28/08/2024 13:09, Boris Brezillon wrote:
Hi Mihail,
On Thu, 8 Aug 2024 12:41:05 +0300
Mihail Atanassov <mihail.atanassov@xxxxxxx> wrote:
+/** + * struct drm_panthor_timestamp_info - Timestamp information +
* + * Structure grouping all queryable information relating to the
GPU timestamp. + */ +struct drm_panthor_timestamp_info { + /**
@timestamp_frequency: The frequency of the timestamp timer. */ +
__u64 timestamp_frequency; + + /** @current_timestamp: The current
timestamp. */ + __u64 current_timestamp;
As it stands, this query has nothing to do with the actual GPU so
doesn't really belong here.
It'd be more valuable, and can maybe give better calibration results
than querying the system timestamp separately in userspace, if you
reported all of:
* the system timer value
* the system timer frequency
* the GPU timer value
* the GPU timer frequency (because it _could_ be different in some
systems)
Duh, I wish this wasn't the case and all SoC vendors went for the
arch-timer which guarantees the consistency of the timestamp on the GPU
and CPU. But let's say this is a case we need to support, wouldn't it
be more useful to do the CPU/GPU calibration kernel side (basically at
init/resume time) and then expose the formula describing the
relationship between those 2 things:
CPU_time = GPU_time * GPU_to_CPU_mul / GPU_to_CPU_div +
GPU_to_CPU_offset;
TIMESTAMP_OFFSET should indeed be set by the kernel (on resume). But I
don't think we need to post M/D+offset to userspace. The 2 Frequencies +
the scalar offset are the raw sources, and userspace can work back from
there.
Sure. No matter how you express the relationship, my point was, if the
calibration is supposed to happen in the kernel at resume time,
returning both the CPU/GPU time in DEV_QUERY_TIMESTAMP to make sure the
sampling is close enough that they actually represent the same
timestamp might not be needed, because you can easily convert from one
domain to the other.
I think it makes more sense after reading [1] :-). This being said, the
maxDeviation is here to account for any latency that might exists
between each domain sampling, so I'd be tempted to read the CPU
monotonic time through the regular syscalls rather than add it to the
DEV_QUERY_TIMESTAMP ioctl.
Wouldn't that defeat the point of getting low-latency consecutive reads
of both time domains? If you leave it to a separate syscall, you're at
the mercy of a lot of things, so it's not just a scalar time delta,
you'll get much higher measurement variance. Doing it in-kernel with no
sleeps in the middle gets you better confidence in your samples being
consistently correlated in time. If you have that in-kernel low latency
correlation pairwise for all time domains you're interested in (in this
case CPU & GPU timestamps, but you could get CPU & display IP
timestamps, etc), you can then correlate all of the clocks in userspace.
Fundamentally, though, if you don't report CPU timestamps in v1 of the
ioctl, you can extend later if it becomes clear that the maxDeviation is
not low enough with the samples being across a syscall.
[1]https://docs.vulkan.org/features/latest/features/proposals/VK_EXT_calibrated_timestamps.html
--
Mihail Atanassov <mihail.atanassov@xxxxxxx>
